1. Field of the Invention
The present invention relates to an exhaust gas cleaning device constructed of a cylindrical metal casing and a metal-made honeycomb core body for carrying an exhaust gas cleaning catalyst. As a cleaning means for exhaust gas from a motor vehicle, the exhaust gas cleaning device is generally installed at an intermediate point of an exhaust pipe.
More specifically, this invention is concerned with an exhaust gas cleaning device of the above sort, which is employed under severe conditions. The exhaust gas cleaning device features the use of a metal casing with improved resistance to deformations and breakage by thermal expansion and stresses.
2. Description of the Related Art
Exhaust gas cleaning devices of the above sort have conventionally been fabricated in the following manner. Namely, a planar band made of a heat-resistant steel sheet and a corrugated band obtained by forming a similar steel sheet into a wavy or sinuous form are superposed one over the other in a contiguous relationship. They are rolled into a multi-layered spiral form, thereby forming a honeycomb-shaped multi-layered composite body axially defining a number of network-patterned gas flow passages for allowing exhaust gas to flow therethrough. Needless to say, the planar band and corrugated band can be superposed as layers one over the other in a contiguous relationship, thereby forming a honeycomb-shaped multi-layered composite body axially defining a number of network-patterned gas flow passages through which exhaust gas is allowed to flow. These honeycomb-shaped multi-layered composite bodies will hereinafter be called "honeycomb core bodies".
A honeycomb core body formed as described above is then inserted into a cylindrical metal casing which is open at both ends thereof. The honeycomb core body and metal casing are thereafter firmly joined together by brazing or soldering, so that they can withstand thermal stresses and deformations due to the high temperature of exhaust gas, exothermic reactions between exhaust gas and the catalyst and the like and further vibrations while the associated motor vehicle is running. Needless to say, the planar and corrugated bands which make up the honeycomb core body are firmly joined together at contacts therebetween by any one of various suitable methods, for example, by welding or brazing.
In an exhaust gas cleaning device constructed of a honeycomb core body of the above sort and a cylindrical casing, an outer peripheral surface of the metal casing is exposed to the outside air so that heat of the honeycomb core body is radiated from the outer peripheral surface of the metal casing. A considerable temperature difference therefore occurs between the outer peripheral wall of the metal casing and the internal honeycomb core body. The flow velocity of exhaust gas is faster through an axially central portion. Accordingly, the temperature of the axially central portion becomes higher than that of a peripheral core portion due to reactions of exhaust gas, which reactions are induced by a catalyst carried on the honeycomb core body. As a result, thermal stresses from the honeycomb core body are concentrated on an outer peripheral wall of the metal casing. Namely, deformation forces and strains which are caused by thermal expansion and thermal stresses which occur in and/or between members (i.e., planar band and corrugated band) making up the honeycomb core body are concentrated on the outer peripheral wall of the metal casing. This may lead to cracking and breakage of the outer peripheral wall of the metal casing and therefore is very dangerous. Moreover, this may result in a reduction to the service life of the exhaust cleaning device.